How to get the value of the cell above to use in math calculation

[rob_k1]

Hi,

I am trying to write an expression for a column that uses the value of the cell above if it exists and if its a number.  Here is the AppSheet translation of the working Google Sheets formula.  

IF(ISNUMBER(R[-1]C), R[-1]C, 0) - RC[-2] + RC[-1]

Is something like this possible?  I would be fine leaving it in Google Sheets but the table breaks if a row is deleted since it doesn't delete the actual row in Google Sheets and only deletes the values.  

The gist is that a record either removes something from an inventory number or adds to it.  

Thank you!